We have recently reported the role of cb1 protein, a tyrosine kinase negative regulator, in the degradation of a number of protein tyrosine kinases and tyrosine phosphorylated substrates. It was further shown that the cb1 and its family members have a RING-type E2 dependent E3 ubiquitin protein ligase activity Experiments are proposed to investigate the mechanism of ubiquitin-mediated down regulation of activated receptor protein-tyrosine kinases and other tyrosine phosphorylated proteins by cb1 family E3 ubiquitin ligases. Another class of E3 ubiquitin ligases which catalyzes the transfer of ubiquitin directly to substrate contains HECT domain (homologues to E6-AP C-terminus). Experiments are proposed to investigate the HECT domain of a 100 kD lipid binding protein WWP1. In addition to structure function studies, attempts will be made to identify the phosphorylated and unphosphorylated target proteins that interact with four WWP1WW domains. Finally studies are proposed to study the role of RING finger domain in BRCA1 tumor suppressor proteins in ubiquitin-mediate protein degradation. Both biochemical and biological assays are proposed to study if the RING finger domains is essential for BRCA1 function."